Microfiber webs of the invention have a partial similarity to calendered constant-thickness microfiber webs of the type suggested in Francis, U.S. Pat. No. 2,464,301, and Prentice, U.S. Pat. No. 3,704,198. In these calendering procedures, the webs are pressed with a heated platen or roll having projections that cause the webs to be compacted and fused together at spaced locations. The result is a plural-density web, with the areas of high density being intended to strengthen the web.
However, microfiber webs of the present invention are unique over such prior-art calendered webs both in structure and utility. Whereas a calendering operation increases the density and reduces the volume of a conventional flat web, procedures of the present invention generally provide a lower density and a greater volume than exhibited by such a conventional flat web. That is, pillowed regions of microfiber webs of the invention have an expended nature because they are collected over an opening. Similarly, collection over an opening gives rise to displacement of the pillowed regions above the level of the compacted regions in the manner shown in FIG. 1, a structure that does not result from calendering of a constant-thickness web.
Thus, while microfiber webs of the invention have the increased tensile strength and integrity that is the object of calendering, at the same time they have a lower density and greater volume and surface area. The result is an increased utility, as previously described.
Another background reference is an article by R. R. Buntin and D. T. Lohkamp entitled "Melt-Blowing -- A One-Step Process for New Nonwoven Products," TAPPI, Volume 56, No. 4, pp. 74-77, reportedly presented as a paper on Oct. 23-24, 1972. In this article it is suggested that microfibers can be collected on a "patterned" surface in such a way as to cause the web to conform to the collector surface, and thereby form a variety of "dimpled or waffle-patterned webs." A fundamental difference between my procedure and the procedure described in the article is that in my procedure, microfibers are collected over open areas of a collection screen so that the microfibers penetrate into the openings. The low-density pillows formed by this penetration are unlike any structure that is formed when microfibers are collected on a surface.
Other background references are the many prior-art teachings directed to preparation of microfiber webs, including such publications as Report No. 4364 of the Naval Research Laboratories, published May 25, 1954, entitled "Manufacture of Superfine Organic Fibers," by Wente, Van A.; Boone, C. D.; and Fluharty, E. L.; and a more brief discussion in Wente, Van A., "Superfine Thermoplastic Fibers," in Industrial Engineering Chemistry, Volume 48, page 1342 et seq (1956); and such patents as Francis, U.S. Pat. No. 2,483,406; Ladisch, U.S. Pat. No. 2,612,679; Till et al, U.S. Pat. No. 3,073,735; and Mabru, U.S. Pat. No. 3,231,639. None of these prior-art teachings contemplates formation of low-density pillowed microfiber webs such as the webs of the inventions.
In summary, insofar as I am aware, the prior art has never contemplated microfiber webs such as the unique pillowed microfiber webs of the invention, with their combination of low density, high volume, high exterior surface area, and good web integrity. Nor, insofar as I am aware, has the prior art contemplated the preparation of such plural-density webs by a direct single-step collection procedure, which requires no further steps such as calendering after the collection operation is completed.